Piezoelectric sensor and acceleration sensor

ABSTRACT

A piezoelectric sensor has a piezoelectric element having a specific axis of sensitivity and a package for the piezoelectric element. The package has a rectangular parallelopiped configuration with opposite longitudinal end surfaces having a height-to-width ratio of about 1:1. External lead electrodes are formed to cover at least the longitudinal end surfaces. A method and apparatus for detecting if the sensor is disposed in the proper posture is also disclosed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a piezoelectric sensor forsensing acceleration or vibration and, more particularly, to apiezoelectric sensor to be mounted on a substrate such as a printedcircuit board.

[0003] 2. Description of the Related Art

[0004] An acceleration sensor which is a kind of piezoelectric sensor isusually used by packaging it or mounting it on a mounting substrate suchas a circuit board which has a circuit for processing accelerationsignals generated by the sensor.

[0005] Mounting of an acceleration sensor on a mounting substrate hascaused two manor problems in the prior art.

[0006] The first problem is as follows. The design of an accelerationsensor to be mounted on a substrate is made such that the sensor sensesacceleration acting in directions perpendicular to the plane of themounting substrate or acceleration acting parallel to the plane of themounting substrate, depending on factors such as the use of the sensorand specifications of the circuit board. It is therefore necessary thatthe acceleration sensor be mounted on the circuit board so as to besensitive to acceleration acting in either one of the above-mentioneddirections.

[0007] Meanwhile, the acceleration sensor has a rectangularparallelopiped configuration having six surfaces, and is adapted to bejointed to the circuit board at a specific one of the six surfaces or asurface opposite to this surface. This is because the accelerationsensor has a flattened rectangular parallelopiped configuration having arectangular bottom and top surfaces and side surfaces of a height whichis small as compared with the two sides of the rectangle comprising thetop and bottom surfaces, so that the sensor can be soldered to thecircuit board while being stably held on the circuit board. Forinstance, the height ranges from 0.5 to 0.7 times as large as the lengthof the shorter side of the rectangle defining the top and bottomsurfaces.

[0008] Consequently, it has been necessary to prepare two types ofacceleration sensors: an acceleration sensor of the type which iscapable of sensing acceleration acting perpendicularly to the plane ofthe circuit board and an acceleration sensor of the type which iscapable of sensing acceleration acting parallel to the plane of thecircuit board. An acceleration sensor of either one of these two typesis selected for use. This not only raises the costs of production ofacceleration sensors but also requires costs for storing andadministrating these two types of acceleration sensors, resulting in arise in the costs of various products incorporating such accelerationsensors.

[0009] The second problem is as follows: The circuit board on which anacceleration sensor is packaged may be deflected for any reason, beforeor after the packaging of the acceleration sensor. Specifically, thedeflection of the circuit board causes a corresponding deformation ofthe acceleration sensor. Such a deformation of the acceleration sensordue to an external force adversely affects the piezoelectric memberwhich senses acceleration, thus hampering sensing of acceleration. Thisproblem is particularly serious when the acceleration sensor is intendedto sense acceleration acting perpendicularly to the plane of themounting substrate, because in such a case the direction of accelerationto be sensed coincides with the direction of deflection of the mountingsubstrate. Consequently, the acceleration sensor may be influenced bythe deflection of the circuit board so as to erroneously produce anacceleration signal even when there is no acceleration acting on thesensor.

SUMMARY OF THE INVENTION

[0010] Accordingly, an object of the present invention is to provide apiezoelectric sensor which can reduce adverse effects due to thedeflection of the substrate and a piezoelectric sensor which can bemounted on a circuit board selectively either in a direction to senseacceleration perpendicular to the plane of the circuit board or in adirection to sense acceleration parallel to the plane of the circuitboard, thereby obviating at least one of the two major problemsencountered with the prior art.

[0011] To this end, according to one aspect of the present invention,there is provided a piezoelectric sensor comprising: a piezoelectricelement having a specific axis of sensitivity; and a package in whichthe piezoelectric element is packaged; wherein the package has arectangular parallelopiped configuration with end surfaces having aheight-to-width ratio approximating 1:1, and wherein external leadelectrodes are formed on at least the end surfaces.

[0012] According to a second aspect of the present invention, there isprovided an acceleration sensor comprising: a bimorph element having anaxis of highest sensitivity extending in a direction which substantiallycoincides with a line normal to the plane of a circuit board; and a caseassembly for fixing and supporting both longitudinal ends of the bimorphelement, the case assembly being adapted to be mounted on the circuitboard at both its longitudinal ends which support both longitudinal endsof the bimorph element; wherein the bimorph element has a pair ofpiezoelectric ceramic plates each having a signal electrode and anintermediate electrode formed on the opposite major surfaces thereof,the piezoelectric ceramic plates being joined to each other face to faceat their surfaces having the intermediate electrodes such that theintermediate electrodes are coupled to each other; each of thepiezoelectric ceramic plates being sectioned in the longitudinaldirection of the bimorph element into three sections including a centralsection and both end sections at border lines which are positioned suchthat when the bimorph element is deformed in response to deflection ofthe circuit board, the quantity of charges generated in the centralsection equals the sum of the quantities of the charges generated inboth the end sections, the central section and both end sections of eachpiezoelectric ceramic plates being polarized thicknesswise of thepiezoelectric ceramic plate in opposite directions, the directions ofpolarization of the central section and both end sections of one of thepiezoelectric ceramic plates being opposite to those of the other of thepiezoelectric ceramic plates.

[0013] According to a third aspect of the present invention, there isprovided an electronic part having a polyhedral body on opposing endsurfaces of which are formed electrodes for outputting voltages ofdifferent polarities, the electronic part comprising: a conductive filmformed on one of the surfaces of said body orthogonal to the endsurfaces having the electrodes at a predetermined position closer to oneof the end surfaces than to the other, the conductive film having anarea large enough to be simultaneously contacted by a pair of probeterminals for applying a voltage.

[0014] According to a fourth aspect of the present invention, there isprovided a method of examining posture of an electronic part which hasto be placed in a predetermined posture in terms of up and down, leftand right and front and back directions, the method comprising: bringinga pair of probe terminals into contact with a potion of an upwardlydirected surface of the electronic part disposed in an examinationposition in an arbitrary posture, the portion being closer to one of theend surfaces having electrodes than to the other; applying a voltagebetween the probe terminals; and determining, based on the presence orabsence of electrical current between the probe terminals, whether theelectronic part as the examination object has been placed in a correctposture.

[0015] According to a fifth aspect of the invention, there is providedan apparatus for examining posture or an electronic part and which hasto be placed in a predetermined posture in terms of up and down, leftand right and front and back directions, the apparatus comprising: apair of probe terminals adapted to be brought into contact with a potionof an upwardly directed surface of the electronic part disposed in anexamination position in an arbitrary posture, the portion being closerto one of the end surfaces having electrodes than to the other;detecting means for applying a voltage between the probe terminals andfor detecting presence or absence of electrical current between the pairof probe terminals; and determining means for determining, based on theresults of the detection by the detecting means, whether the electronicpart as the examination object has been placed in a correct posture.

[0016] The piezoelectric sensor in accordance with the present inventionis designed to be stably seated on a printed circuit board regardless ofthe posture of mounting thereof on the printed circuit board. Therefore,a single piezoelectric sensor can provide a variety of directions ofaxis sensitivity, by changing the posture of the piezoelectric sensormounted on the printed circuit board, thus widening the adaptability ofthe piezoelectric sensor. The present invention therefore achieves aremarkable reduction in the costs of production and management ofpiezoelectric sensors and, hence, the price of the same.

[0017] The acceleration sensor of the present invention offers anadvantage in that, even if the bimorph element is deformed due to theinfluence of deflection of the circuit board on which the accelerationsensor is mounted, charges generated as a result of the deformation arecanceled by each other, so that no signal charges are derived from thesensor when no acceleration is acting thereon, whereby the influence ofdeflection of the circuit board is eliminated. However, whenacceleration acts on the acceleration sensor, charges are generated inthe central section and both end sections of each piezoelectric ceramicplate, based on the relationship between the directions of polarizationof these sections and the tensile and compression stresses caused by thedeformation. Such charges are picked up as output signal voltage,without being canceled, thus providing a high level of output signal.According to the invention, therefore, it is possible to suppressinfluence caused by deflection of the circuit board, while achieving ahigher degree of reliability. Moreover, a high level of sensor outputcan be obtained when acceleration actually acts on the sensor.

[0018] The electronic part in accordance with the present inventionenables, with simple electronic processing, confirmation of the postureof the electronic part which is to be mounted on a printed circuit boardor to be loaded in a tape carrier correctly in a predetermined posture,with a high degree of accuracy while avoiding increase in productioncost.

[0019] The examination method and apparatus of the present inventionenables the confirmation of the electronic part under the conditiondescribed above. In particular, the examination apparatus offers aremarkable reduction in the installation cost as compared with theconventional examination system which relies upon image processingtechniques.

[0020] The above and other objects, features and advantages of thepresent invention will become clear from the following description ofthe preferred embodiments when the same is read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a partly-sectioned perspective view of an embodiment ofthe acceleration sensor in accordance with the present invention;

[0022]FIG. 2 is an exploded perspective view of the acceleration sensoras shown in FIG. 1;

[0023]FIG. 3 is a schematic illustration of a piezoelectric elementdeformed under action of acceleration;

[0024] FIGS. 4(a) and 4(b) are illustrations of the postures in whichthe acceleration sensor of FIG. 1 is mounted on a printed circuit board;

[0025]FIG. 5 is an illustration of the acceleration sensor of FIG. 1carried by a tape carrier;

[0026] FIGS. 6(a) to 6(d) are perspective views of a second embodimentof the acceleration sensor in accordance with the present invention,showing positioning of the sensor relative to probe terminals;

[0027]FIG. 7 is a block diagram schematically showing the constructionof an examining device used in combination with the acceleration sensorof the second embodiment;

[0028]FIG. 8 is a perspective view of a modification of the accelerationsensor;

[0029]FIG. 9 is a perspective view of another modification of theacceleration sensor;

[0030]FIG. 10 is a perspective view of still another modification of theacceleration sensor;

[0031]FIG. 11 is an illustration of an acceleration sensor carried by atape carrier;

[0032]FIG. 12 is a schematic sectional perspective view of the thirdembodiment of the acceleration sensor;

[0033]FIG. 13 is an illustration of a bimorph element deformed underaction of an acceleration; and

[0034]FIG. 14 is a schematic illustration of an acceleration sensordeformed due to deflection of a circuit board.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] First Embodiment

[0036] A first embodiment of a piezoelectric sensor of the presentinvention, serving as an acceleration sensor sensitive to acceleration,will be described with specific reference to FIGS. 1 to 5. FIG. 1 is apartly-sectioned perspective view of an embodiment of the accelerationsensor in accordance with the present invention. FIG. 2 is an explodedperspective view of the acceleration sensor as shown in FIG. 1. FIG. 3is a schematic illustration of a piezoelectric element deformed underaction of acceleration. FIGS. 4(a) and 4(b) are illustrations of thepostures in which the acceleration sensor is mounted on a printedcircuit board. FIG. 5 is an illustration a tape carrier carrying theacceleration sensor.

[0037] Referring to these Figures, an embodiment of the accelerationsensor 1 has a piezoelectric element 2 of a type known as a “bimorphpiezoelectric element”, a package 3 and external leads 4 a, 4 b. Theacceleration sensor 1 has a sensitivity axis which, as indicated by anarrow, extends in the direction of thickness according to a definitiongiven later of the piezoelectric element 2.

[0038] The piezoelectric element 2 has a pair of strip-shapedpiezoelectric ceramic plates 5, 5 each having on its major surfaces asignal pickup electrode 6 and an intermediate electrode 7, thepiezoelectric ceramic plates 5, 5 being jointed face to face at theirintermediate electrodes 7, 7. The direction of stacking of the pair ofpiezoelectric ceramic plates 5, 5 is referred to as the direction ofthickness of the piezoelectric element 2. Each of the piezoelectricceramic plates 5, 5 is divided in the longitudinal direction into threesections 5 a, 5 b and 5 b at border lines L, L. The arrangement is suchthat a tensile stress or a compression stress acts on each section 5 a,5 b, 5 b in response to acceleration G acting on the sensor. The centralsections 5 a and both end sections 5 b of both piezoelectric ceramicplates 5, 5 are polarized in opposite directions along the thicknessesof the piezoelectric ceramic plates 5, as indicated by arrows A, B andC, D. More specifically, the central sections 5 a, 5 a are polarizedtowards each other as indicated by the arrows A and C, whereas both endsections 5 b, 5 b; 5 b, 5 b are polarized away from each other, asindicated by the arrows B and D.

[0039] The package 3 has a pair of clamping members 8, 9 each having asubstantially square-bottomed U-like form when viewed from one sidethereof, for clamping each longitudinal end portion of the piezoelectricelement 2 from the upper and lower sides of the piezoelectric element 2.The package 3 also has a pair of cover members 10, 11 each having asubstantially square-bottomed U-shaped cross-section, the cover members10, 11 being secured to the left and right ends of the clamping members8, 9 so as to cover the left and right side faces of the piezoelectricelement 2. The package 3 generally has a rectangular parallelopipedconfiguration, with opposing end surfaces having a height-to-width ratioof 1:1, i.e., a square configuration. Representing the height and thewidth of the end surface by L1 and L2 (FIG. 1), respectively, accordingto the present invention, it is preferred that the ratio L1/L2 is set toabout 1.0, i.e., to meet the condition of L1=L2, although the inventiondoes not exclude such a height-to-width ratio L1/L2 of from about 0.9 to1.1.

[0040] In accordance with the present invention, the external leadelectrodes 4 a, 4 b are provided in the form of layers laid on thelongitudinal end surfaces of the package 3 including the longitudinalend surfaces of the piezoelectric element 2. One of the external leadelectrodes 4 a, 4 b is connected to one of the signal pickup electrodes6, 6 formed on the piezoelectric ceramic plates 5, 5, while the other ofthe external lead electrodes 4 a, 4 b is connected to the other of thesignal pickup electrodes 6, 6.

[0041] The operation of the described acceleration sensor 1 is asfollows. It is assumed that the central portion of the piezoelectricelement 2 is mainly deflected to be convex upward as illustrated in FIG.3, in response to an acceleration G. In such a case, positive (+)charges are generated on the outer surface of the central section 5 a ofthe piezoelectric ceramic plate 5 which is on the outer side as viewedin the direction of deflection of the piezoelectric element 2, based onthe relationship between the direction A of polarization and the tensilestress Pt. Similarly, positive charges are generated on the outersurfaces of both end sections 5 b, 5 b of the same piezoelectric ceramicplate 5, due to the relationship between the direction B of polarizationand the compression stress Pc.

[0042] The positive charges generated on the outer major surfaces of thecentral section 5 a and both end sections 5 b, 5 b of theabove-mentioned piezoelectric ceramic plate 5 are delivered from thesignal pickup electrode 6 to the associated external lead electrode 4 bwhile being summed with each other.

[0043] In the meantime, negative (−) charges are generated on the outersurface of the central section 5 a of the piezoelectric ceramic plate 5which is on the inner side as viewed in the direction of deflection ofthe piezoelectric element 2, based on the relationship between thedirection C of polarization and the Compression stress Pc. Similarly,negative charges are generated on the outer surfaces of both endsections 5 b, 5 b of the same piezoelectric ceramic plate 5, due to therelationship between the direction D of polarization and the compressionstress Pt. These negative charges are transferred to external leadelectrodes 4 a from the signal pickup electrode 6 associated with thispiezoelectric ceramic plate 5.

[0044] Although negative charges and positive charges are generated onthe inner surfaces of the piezoelectric ceramic plates 5, 5,respectively, the negative charges and the positive charges cancel eachother via the intermediate electrodes 7, 7.

[0045] A description will now be given as to why tension and compressionare applied to the piezoelectric ceramic plates 5, 5 of thepiezoelectric element 2 in response to the action of the acceleration G.When the whole acceleration sensor 1 is accelerated by the accelerationG, such an acceleration directly acts on the package 3 tending to movethe package 3 in the direction of the acceleration G. However, thepiezoelectric element 2 is not directly subjected to such anacceleration so that the piezoelectric element 2 tends to remain in thestate before the application of the acceleration. Namely, inertia forceis generated in response to the acceleration G so as to act on thepiezoelectric element 2. The end sections 5 b, 5 b of both piezoelectricceramic plates 5, 5 tend to move together with the package 3 by beingpulled by the package 3, whereas the central sections 5 a, 5 a tend toremain at the instant position, whereby the central portion of thepiezoelectric element 2 is mainly deformed to deflect in the directionof action of the acceleration G, such as to be convex upward in theillustrated case. As a consequence, tensile stress Pt is generated inthe central section 5 a of the piezoelectric ceramic plate 5 which is onthe outer side of the deflection, i.e., the upper piezoelectric ceramicplate 5, while compression stress Pc is generated in each of the endsections 5 b of the same piezoelectric ceramic plate 5, as will be seenfrom FIG. 3. Conversely, the piezoelectric ceramic sheet 5 which is onthe inner side of the deflection, i.e., the lower piezoelectric ceramicplate 5, is stressed such that compression stress Pc appears in thecentral section 5 a, while tensile stress Pt appears in each of the endsections 5 b, 5 b of this piezoelectric ceramic plate 5.

[0046] The acceleration sensor 1 which has been described is mounted ona printed circuit board 12 such that one of the four major surfaces isdirected upward, depending on the direction of sensitivity axisaccording to the design requirement, in a manner as shown in FIG. 4(a)or a manner as shown in FIG. 4(b). Thus, the direction of thesensitivity axis is determined by the posture of the piezoelectricsensor 1 on the circuit board. Turning sideways or other movement of theacceleration sensor 1 during soldering due to lack of stability isavoided regardless of the posture of the piezoelectric sensor. In FIGS.4(a) and 4(b), numerals 1 a and 1 b denote upper and lower surfaces,while numerals 1 c and 1 d represent right and left side surfaces. Whenthe piezoelectric sensor 1 is mounted in a manner shown in FIG. 4(a),the axis of sensitivity extends in the direction parallel to the printedcircuit board 12 as indicated by the arrow, whereas, when the same ismounted in the posture as shown in FIG. 4(b), the axis of sensitivityextends in the direction perpendicular to the plane of the printedcircuit board 12.

[0047] A process for packaging the acceleration sensor 1 typicallyemploys a tape carrier 20. FIG. 5 shows such a tape carrier 20 by way ofexample. The tape carrier 20 is adapted to hold a plurality ofindependent acceleration sensors 1 of the type described above. Morespecifically, the tape carrier 20 has an embossed tape 21 havingsubstantially square recesses 23 approximating the configuration of theacceleration sensor 1, at a predetermined pitch along the lengththereof, and an upper tape 22 which is bonded to the upper side of theembossed tape 21 so as to close the above-mentioned recesses 23. All theacceleration sensors 1 on the same tape carrier 20 are disposed in thesame posture in regard to the axis of sensitivity as indicated byarrows, depending on the direction of sensitivity axis to be obtainedwhen these sensors 1 are mounted on circuit boards or the like. Forinstance, the posture in which the acceleration sensors 1 areaccommodated in the recesses 23 of the tape carrier 20 is determinedsuch that the left side surface id of each acceleration sensor 1 isexposed through an opening of the recess 23 in the tape carrier 20.

[0048] Preparation of a plurality of acceleration sensors on the tapecarrier 20 in the described manner facilitates packaging of the sensors1 on the printed circuit boards. Namely, the packaging can be performedby a simple process having the steps of peeling the upper tape 22 offthe tape carrier 20 so as to expose one surface of the accelerationsensor 1 in each recess 23, picking up the acceleration sensor 1 by, forexample, a vacuum sucker (not shown) acting on the exposed surface ofthe acceleration sensor 1, and placing the acceleration sensor 1 on thetarget position on the printed circuit board.

[0049] The described embodiment is only illustrative and various changesand modifications may be made thereto. For instance, the piezoelectricelement 2 may be a so-called uni-morph type element which iscantilevered by the clamping members 8, 9 at its one end, rather thanbeing supported at both its longitudinal ends as in the describedembodiment. Furthermore, the piezoelectric element 2 may be constructedto serve as a vibration sensor, although an acceleration sensor has beenspecifically mentioned.

[0050] In the illustrated embodiment, each external lead electrode 4 isformed to cover not only each end surface of the package but also theportion of the peripheral surface of the package near each longitudinalend of the package 3. The external lead electrode 4, however, may beformed to cover only each end surface of the package 3.

[0051] Furthermore, the directions of polarizations of the sections ofthe piezoelectric ceramic plates 5 may be determined such that thedirections A and C of polarizations of the central sections 5 a, 5 a areaway from each other, while the directions B and D of polarizations ofthe end sections 5 a, 5 a; 5 a, 5 a are towards each other.

[0052] Second Embodiment

[0053] The acceleration sensor 1 described as the first embodiment ofthe present invention has a square cross-section perpendicular to thelongitudinal axis. It is therefore impossible to discriminate thedirection of the sensitivity axis based on the appearance. This poses arisk that the acceleration sensor 1 when mounted on a printed circuitboard, for example, is placed in a wrong posture. Such a risk can beovercome when the acceleration sensor 1 of the first embodiment ismounted on the circuit board in accordance with a method which will bedescribed hereinunder. With this method, it is possible to mount theacceleration sensor 1 such that the sensitivity axis is extended in thedesired direction without fail, so that the advantage of theacceleration sensor, which resides in alternative posture of mounting,can be fully enjoyed.

[0054] Although the acceleration sensor of the first embodiment isspecifically mentioned in the following description, it is to be notedthat the following method can be carried out by using other types ofelectronic parts. FIGS. 6(a) to 6(d) are perspective views of theacceleration sensor, while FIG. 7 is a block diagram schematicallyshowing the construction of an acceleration sensor examining device.

[0055] Although not shown, the acceleration sensor 1 has a piezoelectricelement arranged in a manner like a bridge and packaged in an insulatingpackage which has a rectangular parallelopiped configuration. Thedirection of the axis of sensitivity varies as indicated by arrowsdepending on the posture of the piezoelectric element in the sensor.When the acceleration sensor is placed in the posture as shown in FIG.6(a), the axis of sensitivity extends in the horizontal direction.

[0056] External lead electrodes 32, 33 for delivering voltages ofopposite polarity, i.e., positive and negative voltages, are provided inthe form of layers laid on both longitudinal ends of the accelerationsensor 1. More particularly, the external lead electrode 32 is laid onthe front end surface 31 a of the acceleration sensor 1 and the regionsof the upper, lower, left and right surfaces 31 c to 31 f adjacent tothe front end surface 31 a of the sensor 1. Likewise, the external leadelectrode 33 is laid on the rear end surface 31 b and the regions of theupper, lower, left and right surfaces 31 c to 31 f adjacent to the rearend surface 31 b of the sensor 1. More specifically, the external leadelectrode 32 a on the front end surface 31 a delivers a voltage ofpositive polarity, while the external lead electrode 33 on the rear endsurface 31 b delivers a voltage of negative polarity. A dummy electrode34 formed of a web-like conductive film is provided on the upper surface31 c among the surfaces 31 c to 31 f, at a predetermined position closerto the positive external lead electrode 32 than to the negative externallead electrode 33. The dummy electrode 34 extends over the entire widthof the upper surface 31 c and has a size large enough to allow a pair ofprobe terminals 36 to contact therewith for the purpose of applying avoltage which will be described later. The dummy electrode 34 serves asa sign indicating that the acceleration sensor 1 should be mounted on,for example, a printed circuit board (not shown) in such a posture thatthe surface in which the dummy electrode 34 is present is directedupward, and the surface on which this dummy electrode is provided isdetermined depending on factors such as the direction of the sensitivityaxis, polarities of the external lead electrodes 32, 33, and so forth.The dummy electrode 34 may be formed, for example, by firing withsilver, application of a conductive paste or by plating.

[0057] Whether the acceleration sensor 1 has been situated in correctposture is examined before the sensor 1 is packaged on, for example, aprinted circuit board. The examination is conducted by using anexamination device 35 which will now be described with reference to FIG.7.

[0058] The examination device 35 has a pair of probe terminals 36 forapplying a voltage, locating/actuating device 37 for locating the probeterminals 36 both in vertical and horizontal directions, voltageapplication circuit 38 for applying a predetermined voltage to the probeterminal 36, detector 39 for detecting any electrical current betweenthe probe terminals 36, discriminator 40 for discriminating whether ornot the acceleration sensor 1 is in correct posture, based on theresults of detection performed by the detector 39, display 41 fordisplaying the results of the discrimination performed by thediscriminator 40, and control circuit 42 for controlling the operationsof the described components in order to execute the examinationprocessing. The detector 39, which is in this embodiment acurrent-sensitive detector, may be of the type which senses resistanceor capacitance. When such a detector is used, the discriminator 40performs the discrimination based on the results of comparison betweenthe detected value and a predetermined reference value.

[0059] Using the examination device 35 having the describedconstruction, an examination as to whether the acceleration sensor 1 isin correct posture or not is examined in accordance with the followingprocedure.

[0060] The acceleration sensor 1 as the examination object is placed ina predetermined posture in regard to up and down, left and right andfront and back directions. When the posture of the acceleration sensor 1is correct, the upper surface 31 c, i.e., the surface carrying the dummyelectrode 34, is directed upward and, when the sensor 1 in such aposture is viewed in top plan view, the front end surface 31 a isdirected to the south and the rear end surface 31 b is directed to thenorth, with the left and right side surfaces 31 e and 31 f respectivelydirected to the east and west. Thus, the dummy electrode 34 ispositioned closer to the south end of the sensor than to the north end.

[0061] The probe electrodes 36 are brought into contact with theacceleration sensor 1. Namely, the locating/actuating device 37 isactivated to move the probe electrodes 36 to an area where theacceleration sensor 31 is disposed. More specifically, the probeelectrodes 36 are moved to and located at a position where the dummyelectrode 34 should exist when the acceleration sensor 31 is disposed inthe correct posture, and are further moved into contact with the dummyelectrode 34.

[0062] Then, a predetermined voltage is applied between the probeelectrodes 36 across the sensor 1 so as to detect any electrical currentflowing between the probe electrodes 36.

[0063] If the acceleration sensor 1 is in the correct posture as shownin FIG. 6(a), the probe electrodes 36 safely contact with the dummyelectrode 34 so that an electrical current flows between the probeelectrodes 36 through the dummy electrode 34 so as to be detected by thedetector 39. In response to the output from the detector 39, thediscriminator 40 determines that the acceleration sensor 1 is in thecorrect posture, and the control circuit 42 gives instruction to thedisplay 41 to cause the latter to display that the examined accelerationsensor 1 is in the correct posture.

[0064] However, if the acceleration sensor 1 is in a wrong posture,e.g., turned upside down, falls sideways or rotated such that the frontend surface is directed improperly, as shown in FIGS. 6(b) to 6(d), noelectrical current flows between the probe electrodes 35 because theseelectrodes do not contact with the dummy electrode 34 on theacceleration sensor 1. When no electrical current is detected by thedetector 39, the discriminator 40 determines that the accelerationsensor 1 is in a wrong posture, so that the control circuit 42 givesinstruction to cause the display 41 to display that the examinedacceleration sensor 1 is in the wrong posture.

[0065] The operator monitors the display on the display 41 to checkwhether each acceleration sensor is in the correct posture. When theacceleration sensor has been placed in the correct posture, the assemblyprocess advances to the next step for soldering, otherwise the processskips to a step in which an operation is performed to correct theposture of the acceleration sensor 1. Although in the describedembodiment, display 41 is employed to display the results of theexamination, such a display operation is not essential. Namely, one ofthe above-mentioned two steps may be automatically selected inaccordance with the output from the discriminator.

[0066] Thus, the examination device 35 is required only to perform atwo-step operation applying a voltage through the probe terminals 36 anddiscriminating presence or absence of electrical current flowing betweenthe probe terminals 36. Thus, the described examination device 35handles much less amount of data as compared with known examinationapparatus which rely on image processing techniques, whereby the costsof the facility for the examination are greatly reduced. In addition,examination is performed with a higher degree of reliability than bymanual inspection and makes it possible to realize an inexpensiveexamination system incorporated in an automatic production line.

[0067] The dummy electrode 34 on the acceleration sensor 1 can be formedsimultaneously with the formation of the external lead electrode 32, 33,without requiring any additional step in the production process, so thatcost of production of the acceleration sensor 1 is not substantiallyraised.

[0068] The described embodiments are only illustrative and variouschanges and modifications may be made thereto.

[0069] Firstly, it is to be noted that the square cross-section of theacceleration sensor is not essential and other cross-sectional shapesalso may be employed. For instance, acceleration sensors asmodifications which will now be described have a rectangularcross-section perpendicular to the longitudinal axis.

[0070] In these modifications, the positive external lead electrode 32and the negative external lead electrode 33 provided on the accelerationsensor 1 are made to have different areas as shown in FIGS. 8 to 10 soas to provide information as to the posture, instead of the dummyelectrode 34 used in the described embodiments.

[0071] More specifically, FIG. 8 shows a modification in which theportion of the positive external lead electrode 32 on the upper surface31 c of the acceleration sensor 1 is extended to a region contactablewith the probe electrodes 36, whereas the size and area of the portionsof the external lead electrode 32 on the lower surface 31 d and left andright side surface 31 e, 31 f, as well as the size and area of thenegative external lead electrode 33 on all the surfaces 31 c, 31 d, 31 eand 31 f, are so designed that these portions are not extended toregions contactable with the probe electrodes 36.

[0072] In the modification shown in FIG. 9, the portion of the externallead electrode 32 on the upper surface 31 c is extended to the samelength as that in the first embodiment, but the portions of thiselectrode 32 on the lower surface 31 d and left and right side surfaces31 e, 31 f, as well as the portions of the negative external leadelectrode 33 on all the surfaces 31 c, 31 d, 31 e and 31 f, areretracted so as not to be contacted by the probe terminals 36.

[0073] In the modification shown in FIG. 10, the portion of the positiveexternal lead electrode 32 on the upper surface 31 c of the accelerationsensor 1 has the same size and area as that in the embodiment shown inFIG. 9, whereas the portions of the positive and negative external leadelectrodes 32, 33 on both side surfaces 31 e, 31 f are omitted. Theportion of the positive external lead electrode 32 on the lower surface31 d, as well as the portions of the negative external lead electrodes33 on the upper and lower surfaces 31 c, 31 d, is formed to extend overonly half the width of the acceleration sensor 1 so as not to besimultaneously contacted by two probe electrodes 36.

[0074] Each of the modifications shown in FIGS. 8 to 10 can be examinedby using the examination device 35 described before. Examinations of themodifications shown in FIGS. 9 and 10, however, require that thepositions at which the probe terminals 36 contact the accelerationsensor are shifted from that in the cases shown in FIGS. 6(a)-6(d) and8.

[0075] Although in the described embodiments the external leadelectrodes 32, 33 are formed to cover not only the longitudinal frontand rear end surfaces 31 a, 31 b but also portions of the upper, lower,left and right surfaces 31 c to 31 f adjacent to the respective endsurfaces, the present invention does not exclude such an arrangementthat the external lead electrodes 32, 33 are formed to cover only thefront and rear end surfaces 31 a, 31 b.

[0076] In the embodiment described in connection with FIGS. 6(a) to 10,the examination device 35 is used for the purpose of examining theposture of the acceleration sensor 1 in the course of mounting of thesensor on a printed circuit board. The same examination device 35 alsocan be used for the purpose of examining the posture of the accelerationsensor 1 when the sensor 1 is loaded on a tape carrier 50 as shown inFIG. 11 which is used, as explained before, for the purpose of conveyinga plurality of acceleration sensors 1.

[0077] The tape carrier 50 is adapted to hold a plurality of independentacceleration sensors 1 of the type described above. More specifically,the tape carrier 50 has an embossed tape 51 having substantially squarerecesses 53 approximating the configuration of the acceleration sensor1, at a predetermined pitch along the length thereof, and an upper tape52 which is bonded to the upper side of the embossed tape 51 so as toclose the above-mentioned recesses 53. All the acceleration sensors 1 onthe same tape carrier 50 are disposed in the same posture in regard tothe axis of sensitivity, depending on the direction of sensitivity axisto be obtained when these sensors 1 are mounted on a circuit boards orthe like. For instance, the posture in which the acceleration sensors 1are accommodated in the recesses 53 of the tape carrier 50 is determinedsuch that the upper surface 31 c of each acceleration sensor 1 isexposed through the opening of the recess 23 in the tape carrier 50.Loading of the acceleration sensors 1 in the predetermined posture onthe tape carrier 50 can be executed advantageously and effectively byusing the examination device 35 which is capable of ascertaining thatthe acceleration sensors 1 have been placed in the correct posture.

[0078] Third Embodiment

[0079] A third embodiment of the acceleration sensor in accordance withthe present invention will now be described with reference FIG. 12,which is a schematic perspective view of a third embodiment, FIG. 13which is a schematic illustration of deformation of a bimorph elementunder acceleration, and FIG. 14 which is a schematic illustration of theacceleration sensor deformed due to deflection of a circuit board.

[0080] The third embodiment of the acceleration sensor is adapted to bepackaged on the surface of a circuit board 7 after being correctlylocated. The acceleration sensor has a bimorph element 61 having ahighest sensitivity axis S extending in a direction substantially normalto the surface of the circuit board, a pair of end case members 72 whichsupport both longitudinal end portions of the bimorph element 61 byclamping these ends at the upper and lower surfaces of the bimorphelement 61, and a pair of side case members 77 which is integrated withthe end case members 72 while sealing both longitudinal side surfaces ofthe bimorph element 61.

[0081] The bimorph element 61 has a pair of strip-like piezoelectricceramic plates 64, 64 each having a thin-film signal electrode 62 and athin-film intermediate electrode 63 formed on the opposite majorsurfaces thereof, the piezoelectric ceramic plates 64, 64 being joinedtogether at their surfaces having the intermediate electrodes 63. Eachpiezoelectric ceramic plate 64 is divided into three sections along thelength thereof: namely a central section 64 a and both end sections 64b, 64 b at a pair of border lines L, L. The positions of the borderlines L, L are so determined that, when the bimorph element 61 has beendeformed due to a deflection of the circuit board 73, the quantity ofthe charges generated in the central section 64 a equals to the sum ofthe quantities of charges generated in both end sections 64 b, 64 b. Thecentral section 64 a and both end sections 64 b, 64 b have beenpolarized thicknesswise of the ceramic plate 64 in opposite directionsas indicated by arrows A and B. Similarly, the other piezoelectricceramic plate 64 also is sectioned into a central section 64 a and endsections 64 b, 64 b, and these sections are polarized thicknesswise inthe directions opposite to those of the corresponding sections of thefirst-mentioned piezoelectric ceramic plate 64, as indicated by arrows Cand D, respectively. Thus, the central sections 64 a, 64 a of bothpiezoelectric ceramic plates 64, 64 of the bimorph element 61 arepolarized inward, i.e., towards each other, as indicated by arrows A andC in FIG. 12, whereas both end sections 64 b, 64 b; 64 b, 64 b of bothpiezoelectric ceramic plates 64, 64 are polarized outward, i.e., awayfrom each other, as indicated by arrows B and D. One of the pair ofsignal electrodes 62 presented on the outer surfaces of thepiezoelectric ceramic plates 64, 64 is connected to an external leadterminal 78 which covers one longitudinal end of the case assembly 72,77, while the other of the signal electrodes 62 is connected to anexternal lead terminal 78 which covers the other end of the caseassembly 72, 77. The acceleration sensor thus constructed sensesacceleration based on the quantity of charges generated by the bimorphelement 61.

[0082] Deformation of the bimorph element 61 under acceleration mainlyappears in the central section 64 a as schematically shown in FIG. 13.In each of the piezoelectric ceramic plates 64, 64 forming the bimorphelement 13, tensile stress is generated in the central section 64 a,while both end sections 64 b, 64 b sustain compression stress Pc.However, since the central section 64 a and both end sections 64 b, 64 bof each piezoelectric ceramic plate 64 are polarized in oppositedirections, charges generated in the central section 64 a due to tensilestress Pt and charged generated in both end sections 64 b, 64 b due tocompression stress Pc are not canceled by each other, so that thebimorph element 61 produces a large quantity of charges, i.e., a highlevel of output signal.

[0083] More specifically, in the piezoelectric ceramic plate 64 which ison the outer side of the bimorph element 61 as viewed in the directionof the deflection, i.e., the upper piezoelectric ceramic plate 64,positive (+) charges are generated on the outer major surface of thecentral section 64 a based on the relationship between the polarizationdirection A and the tensile stress Pt. Positive charges are alsogenerated on the outer major surfaces of both end sections 64 b, 64 b ofthe same piezoelectric ceramic plate 64, based on the relationshipbetween the polarization direction B and the compression stress Pc.Consequently, the positive charges generated on the outer major surfaceof the central section 64 a and the positive charges generated on theouter major surfaces of both end sections 64 b, 64 b are transferredfrom the signal electrode 62 to the associated external lead electrode78 while being summed with each other.

[0084] In the meantime, negative (−) charges are generated in thepiezoelectric ceramic plate 64 which is on the inner side of theacceleration sensor 1 as viewed in the direction of deflection, i.e., inthe lower piezoelectric ceramic plate 64. More specifically, negative(−) charges are generated in the outer major surface of the centralsection 64 a of this piezoelectric ceramic plate 64, due to therelationship between the polarization direction C and the compressionstress Pc. Negative charges are also generated in the outer majorsurfaces of both end sections 64 b, 64 b of this piezoelectric ceramicplate 64, due to the relationship between the polarization direction Dand the tensile stress Pt. The negative charges generated on the centralsection 64 a and the negative charges generated on both end sections 64b, 64 b are transferred from the signal electrode 62 to the associatedexternal lead terminal 68 while enhancing each other. Consequently,large quantities of positive and negative charges are generated in thebimorph element 61 so that a high level of sensor output is derived fromthis acceleration sensor.

[0085] In each of the piezoelectric ceramic plates 64, 64 underacceleration, charges of polarity opposite to that of the chargesproduced on the outer major surface are generated on the inner majorsurface, i.e., on the surface facing the other piezoelectric ceramicplate 64. These charges on the inner major surfaces of bothpiezoelectric ceramic plates 64, 64 are of opposite polarities and,hence, cancel each other through the electrical connection between theintermediate electrodes 63, 63 on these piezoelectric ceramic plate 64,64.

[0086]FIG. 14 schematically shows the state in which a circuit board 13on which the acceleration sensor is mounted has been deflected or acircuit board 73 after the mounting of the acceleration sensor isdeflected, so as to cause a deformation of the whole bimorph element 61.Note that the deflection of the circuit board 73 deforms the wholebimorph element 61, while the deformation of the bimorph element 61under acceleration mainly appears in the central section 64 a asschematically shown in FIG. 13.

[0087] In such a case, tensile stresses Pt are generated in all thesections 64 a, 64 b, 64 b of the ceramic plate 64 which is on the outerside of the bimorph element 61 as viewed in the direction of thedeflection, while compression stresses Pc are generated in all thesections 64 a, 64 b, 64 b of the piezoelectric ceramic plate 64 which ison the inner side of the bimorph element 61 as viewed in the directionof the deflection. As a result, positive charges and negative chargesare generated on the outer surfaces of section 64 a and sections 64 b,respectively, of the piezoelectric ceramic plate 64 which is on theouter side of the bimorph element 61 as viewed in the direction of thedeflection. Moreover, negative charges and positive charges aregenerated on the outer surfaces of section 64 a and sections 64 b,respectively, of the piezoelectric ceramic plate 64 which is on theinner side of the bimorph element 61 as viewed in the direction of thedeflection.

[0088] As is explained above, since the piezoelectric ceramic plates 64,64 are divided into the central sections 64 a and the end sections 64 b,64 c so that the amount of the charges generated in the central section64 a equals to the sum of the amount of the charges generated in bothend sections 64 b, 64 b, the charges generated in response to thedeformation of the bimorph element 61 cancel each other, so that noelectrical signal is derived from the acceleration sensor based on thedeformation of the bimorph element caused by deflection of the circuitboard. Accordingly, in the case where the bimorph element 61 deflecteddue to the deflected circuit board is also subject to an acceleration,the bimorph element 61 can output signals only in response to theacceleration, and detect the degree of acceleration correctly.

[0089] Although the invention has been described through its specificform, it is to be noted that the described embodiment is onlyillustrative and may be changed or modified within the scope of theinvention which is limited solely by the appended claims.

What is claimed is:
 1. A piezoelectric sensor, comprising: apiezoelectric element having a specific axis of sensitivity; and apackage in which said piezoelectric element is provided; wherein saidpackage has a rectangular parallelopiped configuration with end surfaceshaving a height-to-width ratio approximating 1:1, and wherein externallead electrodes are formed on at least said end surfaces.
 2. Apiezoelectric sensor, comprising: a piezoelectric element having aspecific axis of sensitivity; and a package in which said piezoelectricelement is provided; wherein said piezoelectric element has a pair ofstrip-shaped piezoelectric ceramic plates joined to each other, and saidpackage includes a pair of clamping members for clamping at least onelongitudinal end of said piezoelectric element from upper and lowersides thereof so a to support said piezoelectric element, and a pair ofside cover members secured to both sides of said clamping members so asto cover left and right side surfaces of said piezoelectric element; andwherein said package has a substantially rectangular parallelopipedconfiguration with end surfaces having a height-to-width ratioapproximating 1:1, and external lead electrodes are formed on at leastsaid end surfaces.
 3. An acceleration sensor comprising: a bimorphelement having an axis of highest sensitivity extending in a directionwhich substantially coincides with a line normal to the plane of acircuit board; and a case assembly for fixing and supporting bothlongitudinal ends of said bimorph element, said case assembly beingadapted to be mounted on said circuit board at both its longitudinalends which are in support of both said longitudinal ends of said bimorphelement; wherein said bimorph element comprises a pair of piezoelectricceramic plates each having a signal electrode and an intermediateelectrode formed on opposite major surfaces thereof, said piezoelectricceramic plates being joined to each other face to face at the surfaceshaving said intermediate electrodes such that said intermediateelectrodes are coupled to each other; each said piezoelectric ceramicplate having sections in the longitudinal direction of said bimorphelement, there being three sections including a central section and anend section at each end of the central section, the sections beingpositioned such that, when said bimorph element is deformed in responseto deflection of said circuit board, a quantity of charges is generatedin said central section equal to a sum of the quantities of chargesgenerated in both said end sections, said central section and both saidend sections of each said piezoelectric ceramic plate being polarizedthicknesswise of said piezoelectric ceramic plate in opposite directionsof polarization, the directions of polarization of said central sectionand both said end sections of one of said piezoelectric ceramic platesbeing opposite to those of the other of said piezoelectric ceramicplates.
 4. An electronic part comprising: a polyhedral body havingopposing end surfaces, electrodes being disposed on said opposing endsurfaces for outputting voltages of different polarities, saidelectronic part further comprising: a conductive film formed on onesurface of said body orthogonal to said end surfaces having saidelectrodes at a predetermined position closer to one of said endsurfaces than to the other, said conductive film having an area largeenough to be simultaneously contacted by a pair of probe terminals forapplying a voltage.
 5. An electronic part comprising: a polyhedral bodyhaving opposing end surfaces, electrodes being disposed on said opposingend surfaces for outputting voltages of different polarities, wherein atleast one surface of said polyhedral body orthogonal to said endsurfaces is provided at portions adjacent to said end surfaces withelectrodes formed in continuation from said electrodes on said endsurfaces, one of said electrodes on said at least one surface beingextended to a region contactable by a pair of probe terminals forapplying a voltage, while another electrode on said at least one surfaceis not extended to the region contactable by said pair of probeterminals.
 6. A method of examining posture of an electronic part, theelectronic part comprising a polyhedral body having opposing endsurfaces, electrodes being disposed on said opposing end surfaces foroutputting voltages of different polarities, said electronic partfurther comprising: a conductive film formed on one surface of said bodyorthogonal to said end surfaces having said electrodes at apredetermined position closer to one of said end surfaces than to theother, said conductive film having an area large enough to besimultaneously contacted by a pair of probe terminals for applying avoltage, the electronic part being required to be placed in apredetermined posture in terms of up and down, left and right and frontand back directions, said method comprising: bringing a pair of probeterminals into contact with a portion of an upwardly directed surface ofsaid electronic part disposed in an examination position in an arbitraryposture, said portion being closer to one of the end surfaces havingelectrodes than to the other; applying a voltage between said probeterminals; and determining, based on the presence or absence ofelectrical current between said probe terminals, whether or not theelectronic part has been placed in a correct posture.
 7. A method ofexamining posture of an electronic part, the electronic part comprisinga polyhedral body having opposing end surfaces, electrodes beingdisposed on said opposing end surfaces for outputting voltages ofdifferent polarities, wherein at least one surface of said polyhedralbody orthogonal to said end surfaces is provided at portions adjacent tosaid end surfaces with electrodes formed in continuation from saidelectrodes on said end surfaces, one of said electrodes on said at leastone surface being extended to a region contactable by a pair of probeterminals for applying a voltage, another electrode on said at least onesurface not being extended to the region contactable by said pair ofprobe terminals, the electronic part being required to be placed in apredetermined posture in terms of up and down, left and right and frontand back direction, the method comprising: bringing a pair of probeterminals into contact with a portion of an upwardly directed surface ofsaid electronic part disposed in an examination position in an arbitraryposture, said portion being closer to one of the end surfaces havingelectrodes than to the other; applying a voltage between said probeterminals; and determining, based on the presence or absence ofelectrical current between said probe terminals, whether or not theelectronic part has been placed in a correct posture.
 8. An apparatusfor examining posture of an electronic part, the electronic partcomprising a polyhedral body having opposing end surfaces, electrodesbeing disposed on said opposing end surfaces for outputting voltages ofdifferent polarities, said electronic part further comprising: aconductive film formed on one surface of said body orthogonal to saidend surfaces having said electrodes at a predetermined position closerto one of said end surfaces than to the other, said conductive filmhaving an area large enough to be simultaneously contacted by a pair ofprobe terminals for applying a voltage, the electronic part beingrequired to be placed in a predetermined posture in terms of up anddown, left and right and front and back directions, said apparatuscomprising: a pair of probe terminals adapted to be brought into contactwith a portion of an upwardly directed surface of said electronic partdisposed in an examination position in an arbitrary posture, saidportion being closer to one of the end surfaces having electrodes thanto the other; a detector applying a voltage between said probe terminalsfor detecting presence or absence of electrical current between saidpair of probe terminals; and a circuit for determining, based on theresults of the detection by said detector, whether or not the electronicpart has been placed in a correct posture.
 9. An apparatus for examiningposture of an electronic part, the electronic part comprising apolyhedral body having opposing end surfaces, electrodes being disposedon said opposing end surfaces for outputting voltages of differentpolarities, wherein at least one surface of said polyhedral bodyorthogonal to said end surfaces is provided at portions adjacent to saidend surfaces with electrodes formed in continuation from said electrodeson said end surfaces, one of said electrodes on said at least onesurface being extended to a region contactable by a pair of probeterminals for applying a voltage; another electrode on said at least onesurface not being extended to the region contactable by said pair ofprobe terminals, the electronic part being required to be placed in apredetermined posture in terms of up and down, left and right and frontand back directions, the apparatus comprising: a pair of probe terminalsadapted to be brought into contact with a portion of an upwardlydirected surface, said electronic part disposed in an examinationposition in an arbitrary posture, said portion being closer to one ofthe end surfaces having electrodes than to the other; a detectorapplying a voltage between said probe terminals for detecting presenceor absence of electrical current between said pair of probe terminals;and a circuit for determining, based on the results of the detection bysaid detector means, whether the electronic part has been placed in acorrect posture.